1. Field of the Invention
The present invention relates to a piezoelectric driven MEMS apparatus and a portable terminal having the MEMS apparatus.
2. Related Art
As variable capacitor elements, varicap diodes utilizing a depletion layer thickness change of a P/N junction are conventionally used. In the existing circumstances, however, the varicap diodes are restricted in use, because the capacitance change width which can be obtained is approximately five times at most and the Q value which represents smallness of the loss is as small as approximately 20 to 30.
On the other hand, variable capacitors fabricated by using the MEMS (Micro-electro-mechanical System) technique are drawing the attention recently. A movable electrode (action part) of a variable capacitor is formed on a beam supported in the air over a substrate, and a fixed electrode of the variable capacitor is formed on the substrate opposed to the beam. A variable capacitor is constituted by driving the beam with electrostatic force, thermal stress, electromagnetic force, or piezoelectric force and thereby changing a distance between the action part and the fixed electrode.
In especially a variable capacitor using piezoelectric counteraction as the drive force of the movable beam among the variable capacitors, the spacing between the action part and the fixed electrode can be changed continuously and largely. Therefore, the capacitance ratio can be made large. Furthermore, since air or gas is used as a dielectric, there are many advantages such as a very large Q value.
Furthermore, it is also possible to make the variable capacitor function as a capacitive type switch in a high frequency (RF) region by using the variable capacitor structure as it is and bringing the action part into contact with the fixed electrode via an extremely thin dielectric film. Such a switch formed by using the MEMS technique is also drawing attention because it has both a low resistance and a high insulative isolation property in the off-state as compared with a semiconductor switch.
However, the MEMS variable capacitor using the piezoelectric driven mechanism is supported in the air and has a long and thin beam structure including a piezoelectric layer sandwiched between upper and lower electrodes and a wide electrode structure. Therefore, the problem that the beam is warped upward or downward by slight residual stress in the material of the beam or the action part is serious. In order to cope with this problem, a piezoelectric driven MEMS apparatus having a folded structure is proposed by the present inventors (see. for example, JP-A 2006-87231 (KOKAI) and Kawakubo et al., “RF-MEMS Tunable Capacitor With 3V Operation Using Folded Beam Piezoelectric Bimorph Actuator,” Journal of Microelectromechanical systems, Vol. 15, No. 6, December 2006, pp. 1759-1765). The piezoelectric driven MEMS apparatus having the folded structure includes a first beam having a first end fixed to a substrate, a second end serving as a connection end, and a piezoelectric film sandwiched between a pair of electrode films, a second beam having a first end serving as a connection end, a second end serving as an action part, and substantially the same structure and dimension as the first beam, and extending in a direction opposite to that of the first beam from the connection end, and a fixed electrode provided on the substrate so as to be opposed to the action part. In other words, two beams having the same structure and shape are disposed in parallel and ends of them are connected to form the folded structure. Even if the beams are warped due to residual strain at the time of film formation, therefore, the two beams are warped in the same way and it becomes possible to cancel the warps. Therefore, it is anticipated that the distance between the action end of the piezoelectric driven MEMS apparatus and the stationary end fixed to the substrate is kept substantially constant and stable operation becomes possible.
Even if the distance between the action end and the stationary end fixed to the substrate is kept substantially constant, however, the action part connected to a tip of the action end also warps in a paraboloidal form. As a result, an area of contact obtained when the action end comes in contact with a plane-shaped fixed electrode. In addition, even if the action end is pressed against the fixed electrode, the action end is hard to deform because of the paraboloidal shape, resulting in a problem of a small maximum capacitance value. This is one of greatest problems which hinder engineering application of the MEMS variable capacitors.